Electric discharge device



DeC. 16, 1941. v, FRAENCKEL 7 2,266,595

ELECTRIC DISCHARGE DEVICE Filed'Oct. 27, 1939 DISTANCE Inventor-Q VictorH. Frae ckel, by jv f 1W His Attorney.

Patented Dec. 16, 1941 UNiTED STATS OFFICE I 2,266,595 ELECTRICDISCHARGE DEVICE Victor H. Fraenckel, Schenectady, N. Y., assignor toGeneral Electric Company, a corporation of New York The presentinvention relates to electric discharge devices and more particularly tocathode ray tubes and similar devices in which an electron beam isrequired to be projected through an elongated tubular envelope of glassor a similar insulating material.

In the operation of devices of the character referred to diihculty isfrequently encountered due to the occurrence of wall-charging; that isto say, the accumulation of static charges on various portions of theenvelope wall surface. Such charges tend to modify the potentialdistribution along the discharge path and may cause it to departmaterially from the distribution normally maintained by the electrodeelements of the device. An aspect of this phenomenon which is especiallytroublesome in the operation of beam tubes used as high frequencyoscillators and the like lies in the occurrence of sporadic andunpredictable variations from time to time in the amount ofwall-charging, so that unstable and. uncertain operation results.

The difficulties stated in the foregoing may be overcome to a certainextent by arranging conductive electrode elements at the points at whichobjectionable wall-charging tends to occur. In many cases, however, thisexpedient is inconvenient in that the presence of the electrode elementsand their lead-in connections interferes with the intended operation ofthe device as a whole.

It is an object of my present invention to provide means for avoidingthe objectionable consequences of wall-charging by the use ofnonconductive agencies. According to the invention this object is servedby applying to the wall surface in question a finely divided insulatingsubstance of a class which is typified by magnesium oxide, and thegeneric scope of which is indicated in the following.

The features which I desire to protect herein are pointed out withparticularity in the appended claims. The invention, itself, togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. 1 shows in partial section adischarge device suitably embodying the invention and Fig. 2 is agraphical rep- 'resentation useful in explaining the invention.

Referring particularly to Fig. 1, I have chosen to illustrate myinvention in connection with a discharge device adapted to be used as anamplifier for ultra high frequencies. The amplifier itself, apart fromthe particular improvement to be described herein, is the invention ofW. C. Hahn and is fully disclosed and claimed by him in his application,S. N. 211,124 filed June 1, 1938 and assigned to the same assignee asthe present invention.

The arrangement shown comprises an electron beam tube of the cathode raytype which includes an evacuated envelope having an elongated tubularportion [0. This portion, which is of uniform diameter along its length,connects at one end with an enlarged electrode-containing portion II.The envelope is constituted of a low-loss insulating material such asglass or quartz, the latter substances being herein genericallydesignated as vitreous materials.

The tubular envelope portion 10 is provided at one end with means, suchas a known type of electron gun, for producing an electron beam. Thecombination shown comprises a cathode M, which is indicated in dottedoutline, and a focusing cylinder I5 for confining the electrons emittedfrom the cathode to a concentrated beam. The cylinder may either beconnected directly to the cathode as shown, or maintained a few voltsnegative or positive with respect to it. In order to accelerate theelectrons to a desired extent there is provided an acceleratingelectrode l6 which is spaced from the cathode and which may be biased toa suitable positive potential, say, several hundred volts.

At the other end of the envelope there is provided an anode I8, whichserves to collect the electron beam after it has traversed the tubularenvelope portion ID. A ring-like electrode 19 in the nature of asuppressor grid serves to prevent secondary electrons emitted by theanode [8 from returning to the discharge space.

In the operation of the device the anode should be maintained at apotential one to several thousand volts above the cathode and thesuppressor grid 19 should be biased fifty to several hundred voltsnegative with respect to the anode. These potential relationships may beestablished by means of suitable voltage sources conventionallyrepresented as batteries 20 and 2 I. In order to maintain the beam infocus during its passage along the axis of the envelope one may employ aseries of magnetic focusing coils such as are indicated by the numeral23.

The combination of elements so far described comprises means forproducing a unidirectional beam of electrons. Outside the envelope thereis provided an electrode system for modulating the beam, at highfrequency and for abstracting power from the modulated beam.

The electrodes which make up the high frequency system include a seriesof sequentially arranged tubular conductive elements whichconcentrically surround the envelope and which are respectively numbered30 to 36. The tubular elements which have even numbers are solidlyconnected together and may thus be held at a common potential byconnection to the positive terminal of battery 20. The elements 3!, 33and 35 on the other hand, constitute independent electrodes which arecapable of varying in potential with respect to the fixed potentialelements 30, 32, 34 and 36. As is fully explained in the Hahnapplication S. N. 211,124 above referred to, the longitudinal dimensionsof all the tubular elements are, by design, accurately correlated to thevelocity of the electron beam traversing the envelope l and to theparticular frequency at which the device is desired to operate.

In the use of the apparatus as an amplifier, high frequency potential issupplied to the electrode 35 through a concentric conductor transmissionline comprising the conductive elements 4|] and 41. Due to the resultantpotential gradients established at the gaps adjacent the electrodeextremities, longitudinal modulation of the electron beam is produced.This modulation is intensified by the action of electrode 33 and anassociated resonant circuit comprising concentric conductors 43 and 44.It is finally reproduced in amplified form by the reaction of themodulated beam on the electrode 3|. From this electrode the resultantamplified voltage may be applied to output conductors 45 and 41.

In the design of a system such as that described above it is ordinarilyconvenient to start with an intended beam velocity. The dimensions ofthe electrode parts and their spacings are then determined in suchfashion as to be correlated to the assumed velocity. It is apparent,therefore, that in order that the intended operation of the device shallobtain, it is necessary that the beam velocity shall remain at all timesat the assumed value.

To some extent the condition specified in the preceding paragraph can berealized by positioning conductive members 48 and 49 at the bound ariesof the high frequency electrode system and by connecting such electrodesto a voltage which corresponds to the desired beam velocity. It isfound, however, that without additional precautions, the occurrence ofcharging of the wall surfaces of the envelope tends to produceconsiderable variation of the potential distribution between theconducting members and thus to cause a departure of the average beamvelocity from he desired value. The nature of this phenomenon isindicated graphically in Fig. 2, in which the curve A shows thevariation of potential which may occur between the regions a and b (Fig.1), where the potential level is definitely fixed by the presence of theconductive members 43 and 49. Even more important than the mereexistence of a potential variation due to wallcharging is the fact thatthe amount of such variation is not constant with time but tends tochange in a sporadic and unpredictable manner. Consequently, theoperation of the tube I9 as a high frequency device may becomeobjectionably unstable. In some cases the magnitude and frequency ofvariation of the wall-charging may even be such as to cause a spuriousmodulation of the signal output of the device, such modulationcorresponding to a noise component.

While it is theoretically possible to remedy this situation by providingadditional internal electrodes at various points, along the axis of theenvelope, it is frequently inexpedient to do this in connection with anelectrode system of the type illustrated in Fig. 1. The difficultyreferred to arises from the fact that it is objectionableto have bodiesof conductive material arranged within the envelope at points where highfrequency fields existlest the high frequency losses be increased to aninsupportably high value. Moreover, in view of the presence of the highfrequency electrode system, it is difiicult to arrange lead-inconnections for maintaining the intermediate electrode elements at adesired potential level.

In accordance with my present invention the more objectionableconsequences of wall-charging are substantially eliminated in anotherway by coating the insulating surfaces where such charging is apt tooccur with a finely divided non-conductor such as magnesium oxide. Thus,in the present connection, a coating of this kind may be applied asindicated by the stippled area 5!] of Fig. 1. It has been especiallyobserved that the application .of such a coating to a surface of glassor quartz very markedly lessens the occurrence of instability traceableto wall-charg- On the basis of the data now available and the known factthat wall-charging is mainly a secondary emission phenomenon, it isconsidered that the stabilizing effect just referred to may be due tothe circumstance that magnesium oxide and related substances differquite materially from glass and quartz in their secondary emissionproperties. More particularly, it is thought that the higher ratio ofsecondary emission to primary electron current exhibited by thesesubstances tends to increase their potential stability in the presenceof a high velocity electron stream,

In addition to magnesium oxide, other insulating substances of highsecondary emissivity, including beryllium oxide and aluminum oxide maybe alternatively employed. The substance utilized need not be a metallicoxide provided it is of insulating character and possesses secondaryemission characteristics comparable to those of the materials named.

I prefer to use and have particularly referred to magnesium oxide,because of its great ease of application. In this connection it hasproven convenient to develop the-oxide by burning magnesium in air orsome other oxygenous atmosphere and to project the resultant smoke(aerosol) into the vicinity of the glass or quartz surface desired to becoated. By this procedure the magnesium oxide particles may be depositedon and caused to adhere to the glass or quartz in vthe form of anextremely thin film.

A particular virtue of the potential stabilizing coatings described inthe foregoing lies in the fact that their use does not involve theintroduction of conductive masses within the discharge envelope. This isa material advantage in a device such as that shown in Fig. 1, forexample, for the reason that the presence of conductive elements withinthe region bounded by the high frequency electrode system and especiallyin the vicinity of the electrode gaps would tend to produce anobjectionable increase in the R. F. losses of the system. It is alsoadvantageous that the coatings of my invention obviate the necessity forlead-in connections to be provided at inconvenient points as would berequired if potential-fixing electrodes were to be employed.

It will be understood, of course, that the use of the invention is by nomeans limited to devices of the particular character shown in Fig. 1. Onthe contrary, it may be advantageously employed in any kind of beam tubeoperated at a sufficiently high voltage so that instability or otherefiects due to wall-charging are apt to occur. Therefore, while I havedescribed a particular embodiment of the invention, I aim to cover inthe appended claims all such equivalent applications as come within thetrue scope of the preceding disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A discharge device including means for projecting an electron beamalong a path of substantial length, a non-conductive structuresurrounding the beam path and constituted of vitreous material subjectto wall-charging, conductive elements positioned in proximity to theinterior surfaces of said non-conductive structure at spaced pointsalong the beam path, means connecting with said conductive elements forfixing the potential of the said points at a level appropriate to thedesired functioning of the device, and a superficial layer of a highlyinsulating substance covering interior surfaces of said non-conductivestructure which are spaced from said conductive elements and at whichwallcharging tends to occur, said substance being constituted of oxideof the group which includes the oxides of magnesium, beryllium andaluminum and serving the function of stabilizing the potential of thesurfaces to which it is applied.

2. A discharge device comprising an elongated tubular envelope ofvitreous material subject to wall-charging, means for projecting anelectron stream through the envelope, conductive elements positioned inproximity to the interior surfaces of said envelope at spaced pointsalong the envelope axis, means connecting with said conductive elementsfor fixing their potentials at a level appropriate to the desiredfunctioning of the device, and a coating of a finely divided insulatingsubstance applied to the interior surface of the envelope at regionsdisplaced from said conductive elements, said substance beingconstituted of oxide of the group which includes the oxides ofmagnesium, beryllium and aluminum and serving to minimize theobjectionable effects of wall-charging.

3. A discharge device comprising an elongated tubular envelope ofvitreous material subject to wall-charging, means for projecting anelectron stream through the envelope, means including electrode elementslocated at spaced points along the axis of the envelope for fixing thepotential level at such points, and a coating of magnesium oxide appliedto the interior wall surface of the envelope for minimizing theobjectionable effects of wall-charging at regions displaced from saidelectrode elements.

VICTOR H. FRAENCKEL.

